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Desert Diary
Math/Mark and Recapture


Counting a population of animals in a big open space like a desert, is not as simple as standing and counting. Biologists use a mathematical method called "capture-recapture". Let's say you capture 100 snakes in a desert region. You carefully put a tag on them, then set them free. Later, you capture a fresh sample of 100 snakes and find that 20 of them have tags. Under certain assumptions, the proportion of snakes in our second ("recapture") sample that are tagged should equal the proportion of snakes from the total desert population we took our first sample from.

So now let's solve for that unknown total population: 20 out of 100 equals 100 out of what? The answer....: 500 snakes! This capture-recapture technique doesn't apply only to counting animals. Every 10 years the US Census tries to count all the country's humans and then the recapture phase is the Post-Enumerative Survey. I hope this captured your attention, because in the desert, math counts!
pen and ink


Contributor: Lawrence M. Lesser, Mathematical Sciences, University of Texas at El Paso.

Desert Diary is a joint production of the Centennial Museum and KTEP National Public Radio at the University of Texas at El Paso.



1999. Studying Proportions Using the Capture-Recapture Method. Mathematics Teacher, Mar., pp. 215-218.

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